Problem: Triangle $ABC$ has a perimeter of 2007 units. The sides have lengths that are all integer values with $AB< BC \leq AC$. What is the smallest possible value of $BC - AB$?
Since $AB$ and $BC$ are positive integers and $AB < BC$, $BC - AB$ must be at least 1.

The triangle with side lengths $AB = 650$, $BC = 651$, and $AC = 706$ satisfies the given conditions, and for this triangle $BC - AB = 1$.

Therefore, the smallest possible value of $BC - AB$ is $\boxed{1}$.